1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses include an image forming apparatus provided with an image reading unit which reads a surface of paper (paper surface) having undergone image forming.
By using the read image obtained by the image reading unit, misregistration of the formed image can be detected (refer to, for example, Japanese Patent Application Publication No. 2003-211770), and also various types of calibration, such as correction of luminance non-uniformity of the image reading unit, called shading correction, can be performed (refer to, for example, Japanese Patent Application Publication No. 11-127350).
At the time of detection of misregistration of the image, in order to read the paper surface, a background member having a white surface as a reflective plate is arranged at a position facing the image reading unit via the paper surface. On the other hand, at the time of calibration, in order to read a color sample member having a surface of a reference color (s), the color sample member needs to be arranged at the position facing the image reading unit without paper in between.
To switch the background member and the color sample member depending on the reading target, as shown in FIG. 10, a prism F having a background member F1 and a color sample member F2 on lateral surfaces can be used. For example, at the time of detection of misregistration of an image, the prism F is rotated such that the background member F1 is arranged at the position facing an image reading unit 30, and the image reading unit 30 reads the surface of (a sheet of) paper P (paper surface), whereas at the time of calibration, the prism F is rotated such that the color sample member F2 is arranged at the position facing the image reading unit 30, and the image reading unit 30 reads the color sample member F2 between sheets of paper P.
If the paper is cut paper, which has been cut to be a prescribed size of sheets, as shown in FIG. 10, there is a space between sheets of paper P. Therefore, there is a chance to read the color sample member F2 which is arranged under the paper P.
However, if the paper is long paper, such as rolled paper, the paper is continuously conveyed. Hence, unlike cut paper, there is no space between sheets of paper, and even while image forming stops, the paper is on a paper path. Therefore, there is no chance to read the color sample member F2 which is arranged under the paper.